1. Field of the Invention
The present invention relates to a new process for the preparation of polyisocyanates containing isocyanurate groups by the catalytic trimerization of monomeric polyisocyanates using novel trimerization catalysts, and the use of the products obtained by the process according to the invention are isocyanate components for the preparation of isocyanate-polyaddition products.
2. Description of the Prior Art
Catalysts for the trimerization of organic isocyanates, in particular polyisocyanates, are known in large numbers (J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology, page 94 et seq (1962)). Strong organic bases are suitable catalysts for trimerization, e.g. metal salts of carboxylic acids which are alkaline in reaction, metal alcoholates, metal phenolates, alkali metal carbonates, tertiary amines, tertiary phosphines and the "onium" compounds of nitrogen and phosphorus as well as heterocyclic compounds of these elements. The catalysts are frequently used as combinations or together with other compounds acting as co-catalysts, such as mono-N-substituted carbamic acid esters (A. Farkas and G. A. Mills, Advances in Catalyses, Volume 13, 393 (1962)).
In more recent processes known in the art, special organic bases are used as trimerization catalysts, some of which must be prepared by elaborate methods of synthesis.
Thus for example Mannich bases (German Offenlegungsschriften No. 2,551,634 and No. 2,641,380) or tertiary phosphines are used for the trimerization of aromatic polyisocyanates. Where phosphines are used, the uretdiones are first formed, which are converted to the isocyanate in a second reaction phase (German Offenlegungsschrift No. 1,201,992). Organic bases having a certain structure, such as quaternary ammonium hydroxides (EP-A No. 010,589 and EP-A No. 009,694), aminimides (J. E. Kresta, R. J. Chang, S. Kathiriya and K. C. Frisch, Makromol Chem. 180, 1081 (1979)) as well as aziridine derivatives in combination with tertiary amines (DE-AS No. 2,235,826) have recently been frequently used for the trimerization of (cyclo) aliphatic diisocyanates.
Completely novel trimerization catalysts have now surprisingly been found. They are distinguished by the simultaneous presence of numerous remarkable advantages:
1. The new catalysts are suitable for the trimerization of both aromatic and aliphatic polyisocyanates.
2. Most of the new catalysts as well as their reaction products with the catalyst poisons described in more detail below are soluble in the starting materials and end products of the process according to the invention described below, so that laborious methods of separation are unnecessary.
3. The trimerization reaction using the new catalysts may be carried out both solvent-free and in the presence of solvents at comparatively low temperatures so that substantially colorless, clear polyisocyanates containing isocyanurate groups are obtained as end products.
4. The slightly exothermic trimerization reaction can easily be controlled to ensure safety and progress of the process both when carried out continuously and when carried out batch-wise.
5. Only small quantities of catalysts are generally required and consequently also only small quantities of catalyst poisons, but short reaction times are nevertheless obtained.
6. The catalysts are simple and inexpensive to prepare and virtually odorless.